Internal combustion engines (hydrogen-using internal combustion engines) are known which use hydrogen gas as a fuel, in addition to a hydrocarbon base fuel, such as gasoline. Hydrogen gas has higher combustibility than hydrocarbon fuel. Thus, addition of hydrogen gas to hydrocarbon fuel allows the engine to operate in an expanded lean-burn region at a low load, thus providing noticeable effects, for example, improved fuel economy and reduction in the amount of NOx emissions. When the engine operates at a high load, on the other hand, addition of hydrogen gas to hydrocarbon fuel makes it possible to suppress knocking and improve the engine power output to maintain the accelerating ability of the vehicle.
One example of such hydrogen-using internal combustion engines is described in Japanese Laid-open Patent Publication 2003-293809. The hydrogen-using engine described in this publication includes an injector that injects a liquid hydrocarbon fuel and a hydrogen tank connected to the injector via a hydrogen-supply line, and is arranged such that the hydrocarbon fuel and hydrogen gas can be simultaneously injected from the injector in single injection event.
In the above-described hydrogen-using internal combustion engine, two fuel supply lines, namely a hydrogen gas supply line and a hydrocarbon fuel supply line through which hydrogen gas and the hydrocarbon fuel are respectively supplied to the injector, are separately provided. This arrangement complicates the engine system considerably, which reduces efficiency in the installation of the system on the vehicle due to an increased number of components, and increases manufacturing cost, relative to an internal combustion engine that only uses hydrocarbon fuel.
The known hydrogen-using engine as described above uses the hydrogen tank as a means for accommodating hydrogen gas. It is, however, to be noted that hydrogen gas as a gaseous fuel exhibits poor installation or storage efficiency when installed on the vehicle, as compared with a liquid fuel, such as gasoline. In recent years, it has been proposed to produce hydrogen gas from a liquid hydride on the vehicle, so that hydrogen can be stored in a liquid state (in the form of a liquid hydride) having high installation efficiency.
As a method of producing hydrogen gas from a liquid hydride, various methods have been proposed which include, for example, electrolysis, decomposition of the liquid hydride using low-temperature plasma (as described in JP-A-2001-335302), and reduction of the liquid hydride using a metal in a highly active state (as described in JP-A-2004-123517 and JP-A-2003-212501).
In any of the above-indicated methods, a large quantity of electric energy is needed to produce hydrogen gas from a liquid hydride. The electric energy used for producing hydrogen gas may be readily supplied from an on-board (or vehicle-mounted) battery. It is, however, necessary for the engine to drive an alternator so as to charge the battery with electric energy, and, in this case, the amount of fuel consumed by the engine is increased accordingly. Thus, securing electric energy for producing hydrogen gas will be of great importance, in order to achieve high energy-efficiency while yielding advantageous effects from addition of hydrogen to the liquid fuel.
In any of the above-indicated methods of producing hydrogen gas, it takes some time for the produced hydrogen gas to reach the injector. If hydrogen gas is going to be injected from the injector immediately after the engine is started, there is a need to store hydrogen gas in such an amount as to be used immediately after start of the engine. While a tank may be used as a means for storing hydrogen gas, storage of hydrogen gas in the tank is the least desirable in view of the efficiency or easiness with which the tank is installed on the vehicle.